1. Field of the Invention
The present invention relates to a heat sink for cooling an exothermic device, such as a microprocessor, utilizing a heat spreader sheet; a method for forming such a heat sink; and a portable electronic apparatus using such a heat sink.
2. Description of the Related Art
Various electronic devices such as microprocessors are undergoing miniaturization with their operating frequencies being shifted toward higher ranges. With this trend, efficient means for cooling these electronic devices are more and more urgently required. Particularly, a heat sink to be installed in portable electronic apparatuses in which compactness and light weight are required, must be capable of cooling an exothermic device (a device which easily generates heat energy) as efficiently as possible in addition to being light and compact. As a method for answering the first requirement for compactness and light weight, there is a method for transmitting the heat generated in an electronic device to a casing and for cooling the casing by natural air cooling. Because no parts for forced air cooling are necessary in this method, it is possible to provide a heat sink which is smaller and lighter than by using a forced air cooling method.
It is possible to realize a more compact and light weight heat sink using a graphite or graphite composite sheet, as is disclosed in Japanese Patent Laid-Open Publication No. Hei 6-134917 and Japanese Patent Laid-Open Publication No. Hei 8-23183, as a means for transmitting the heat generated in an exothermic device to the device's casing. In addition, graphite and graphite composite materials are also useful for effective cooling, the second requirement for a heat sink of a portable electronic apparatus, because of their high thermal conductivity. FIG. 11 shows a portable electronic apparatus in which a sheet made of graphite or graphite composite materials is used to transmit heat from an exothermic device to a casing. In this figure, exothermic device 1, such as a microprocessor, for example, is mounted on circuit board 2, which is contained in casing 3 of the portable electronic apparatus. Heat spreader sheet 4 made of graphite or graphite composite material is formed by drawing, and assembled into the portable electronic device. Heat spreader sheet 4 is fixed to the inside wall of casing 3 so that the drawn part of heat spreader sheet 4 comes in to contact with exothermic device 1.
However, deep drawing of graphite or graphite composite material is generally difficult because of the material's flexibility. Therefore, when heat spreader sheet 4 must be drawn to come in contact with exothermic device 1, the heat spreader sheet 4 can not be prevented from having a slightly curved shape, as shown FIG. 11. With such a shape, it is difficult to effectively transmit the heat generated in exothermic device 1 to heat spreader sheet 4 because it is difficult for spreader sheet 4 to contact a surface of exothermic device 1. Further, it is difficult to effectively radiate the heat generated in exothermic device 1 to the outside of casing 3 because the distance of heat transmission of heat spreader sheet 4, that is, the distance from exothermic device 1 to the inside wall surface of casing 3, is lengthened. Still further, in order to avoid interference or contact of heat spreader sheet 4 having a gentle slope with adjacent device 5 mounted on circuit board 2, the interval between exothermic device 1 and adjacent device 5 should be somewhat large. As a result, the usable space for mounting electronic apparatus on circuit board 2 is decreased, creating an obstacle to miniaturization of portable electronic apparatus. Further, when an electrically conductive surface such as graphite is exposed on a surface of heat spreader sheet 4, a particularly large interval should be part of the design in order to avoid contact of heat spreader sheet 4 with lead wires of adjacent device 5, in order to avoid a short circuit between casing 3 and the lead wires. When it is impossible for heat spreader sheet 4 to contact exothermic device 1, such a shape becomes unnecessary. However, effective thermal conduction, diffusion, or transfer can not be expected under such a non-contact condition.